First you must consider this important question: does your sister want your help, or does she prefer to to be left in peace by you and your parents?

People do not change their behaviour just because other people regard them as abnormal. They would rather avoid contact, and instead seek contact with people who accept and like them as they are.

How then is it possible to influence a person who lives in unsuitable ways? If you try to be critical and accusing, you will only push her away. She needs to feel that you like her and want to help her. Praise her when she has done something you like. People enjoy getting praise, and praise will cause them to try harder doing what was praised. Do not comment on her mistakes. Talk to her and try to understand her view. Avoid being arrogant and accusing.

Your sister may have an "avoidant personality disorder". This means that she pulls away instead of tackling difficult problems. People with personality disorders see the world in their own peculiar way. Others may find their behaviour provoking, even though they themselves see their behaviour as natural. She will not change just because you think she should change. Her personality is inherited and formed in very early years.

If you want a relation with your sister, you must try to accept her as she is. She needs to feel that you understand and respect her.